For upstream transmission in time-division multiplexing (TDM) passive optical networks (PONs), a common receiver in an optical line terminal (OLT) receives signals, such as a burst, from multiple optical network units (ONUs). Current TDM-PON systems mainly use an on-off-keying (OOK) modulation format, which is sensitive to noise. Other spectrally efficient modulation formats, such as orthogonal frequency division multiplexing (OFDM) and discrete multi-tone (DMT), may alternatively be used to increase the throughput of a PON system.
In conventional OFDM and DMT communications systems, data is modulated on subcarriers or tones used by the system. An inverse fast Fourier transform (IFFT) algorithm is performed on certain modulated tones to obtain a set of time domain samples for transmission, forming an OFDM symbol. DMT is mainly used in transmission systems where OFDM signals are real-valued.